Polymer Composition For Producing Gel Extruded Articles and Polymer Articles Made Therefrom

ABSTRACT

A polymer composition for producing gel extruded articles is described. The polymer composition contains polyethylene particles combined with a plasticizer. The polyethylene particles are particularly selected so that the particles rapidly form a homogeneous gel-like material when combined with the plasticizer during gel processing. In one embodiment, the polyethylene used to produce the particles has a relatively low bulk density. Alternatively or in addition, the particles can have a carefully controlled particle size distribution. Polymer articles such as fibers and films can be produced having little to no imperfections.

RELATED APPLICATIONS

The present application is based on and claims priority to U.S.Provisional Application Ser. No. 62/546,857, having a filing date ofAug. 17, 2017, which is incorporated herein by reference in itsentirety.

BACKGROUND

Polyethylene polymers have numerous and diverse uses and applications.For example, high density polyethylenes are valuable engineeringplastics, with a unique combination of abrasion resistance, surfacelubricity, chemical resistance and impact strength. They findapplication in the production of high strength fibers for use in ropesand anti-ballistic shaped articles and in the production of otherelongated articles, such as membranes for lithium batteries. However,since the flowability of these materials in the molten state decreasesas the molecular weight increases, processing by conventionaltechniques, such as melt extrusion, is not always possible.

One alternative method for producing fibers and other elongatedcomponents from polyethylene polymers is by gel-processing in which thepolymer is combined with a solvent. The resultant gel is extruded into afiber or membrane, and may be stretched in one or two directions. Also,part or all of the solvent may be removed from the product.

In the past, however, problems have been experienced in gel-processingpolyethylene polymers. For example, during gel extrusion of polyethylenepolymers, the appearance of gel specks can compromise the physicalproperties of the resulting product. Gel specks generally refer topolyethylene polymers that have not fully dissolved or otherwiseintimately combined with the solvent during the extrusion process. Whenforming membranes, for instance, these gel specks can render the productunusable for some applications, such as battery separator applications.In the past, in order to remove gel specks, gel processing was done withlower molecular weight polymers which can reduce the strength of theresulting product. Alternatively, processing times can be increased inorder to hopefully eliminate gel specks prior to extrusion. Increasingprocessing time, however, lowers throughput and increases the cost ofthe process.

In view of the above, a need exists for an improved polyethylenecomposition capable of being gel spun or gel extruded at relatively fastspeeds without the formation of gel specks or other impurities. A needalso exists for an improved process for producing extruded articles frompolyethylene polymers using gel-processing.

SUMMARY

In general, the present disclosure is directed to polyethylenecompositions well suited for gel processing applications. Thepolyethylene compositions, for instance, can be used to produceelongated articles, such as films, membranes, fibers, and the like. Inaccordance with the present disclosure, a polyethylene, such as a highdensity polyethylene resin, is combined with a plasticizer to form agel-like material. In accordance with the present disclosure, thepolyethylene resin is particularly selected so as to have a carefullycontrolled bulk density and/or a carefully controlled particle sizedistribution. The polyethylene resin characteristics have been found todramatically improve the rapid formation of a gel-like material withoutany remaining gel specks or impurities that can lead to productimperfections when the composition is extruded.

For example, in one embodiment, the present disclosure is directed to apolymer composition for producing gel extruded articles. The polymercomposition comprises a plasticizer blended with polyethylene resin. Thepolyethylene resin can be made from a high density polyethylene, such asa high molecular weight polyethylene. In one embodiment, for instance,the resin is made from an ultrahigh molecular weight polyethylene. Thepolyethylene resin is combined with the plasticizer in order to producea gel-like composition capable of being extruded. In accordance with thepresent disclosure, the polyethylene resin has at least one of thefollowing characteristics:

-   -   (1) A bulk density of less than about 0.35 g/cm³;        -   and/or    -   (2) A median particle size (d50) of less than 125 microns and        wherein 90% of the particles have a particle size of less than        about 180 microns.        In one embodiment, the polyethylene resin particles include both        of the above characteristics. Polyethylene particles as        described above have been found to dramatically improve gel        processing of the polyethylene polymer. For instance, the        polyethylene particles having at least one of the above        characteristics have been found to rapidly combine with the        plasticizer to produce a homogeneous gel-like material. Thus,        the above composition can be placed into an extruder to produce        a gel-like composition in a very short amount of time and        without the formation of gel specks or other small particles not        dissolved or otherwise homogeneously blended with the        plasticizer.

As described above, in one embodiment, the high density polyethyleneparticles can have a relatively low bulk density for improved blendingwith the plasticizer. The bulk density, for instance, in one embodiment,can be less than about 0.3 g/cm³, such as less than about 0.28 g/cm³,such as less than about 0.26 g/cm³. The bulk density is generallygreater than about 0.15 g/cm³.

Alternatively or in addition to having a low bulk density, thepolyethylene particles can also have a relatively small median size(d50). For instance, the polyethylene particles can have a medianparticle size (d50) of from about 60 microns to less than 125 microns,such as from about 70 microns to about 110 microns. In addition, 90% ofthe high density polyethylene particles can have a particle size of lessthan about 170 microns, such as less than about 165 microns, such asless than about 160 microns, such as less than about 155 microns, suchas less than about 150 microns, such as less than about 145 microns,such as less than about 140 microns.

The high density polyethylene particles of the present disclosure havebeen found to rapidly combine with the plasticizer under heat to form ahomogeneous gel-like material. For example, when tested according to asolubility test as will be described in greater detail below, the highdensity polyethylene particles can have a solubility of less than about3 minutes, such as less than about 2.5 minutes, such as less than about2 minutes.

In general, the polymer composition contains the high densitypolyethylene resin in an amount up to about 50% by weight. Theplasticizer, for instance, can be present in the composition in anamount greater than about 50% by weight, such as in an amount greaterthan about 60% by weight, such as in an amount greater than about 70% byweight, such as in an amount greater than about 80% by weight, such asin an amount greater than about 90% by weight. Various differentmaterials can be used as the plasticizer. For instance, the plasticizermay comprise a mineral oil, a paraffinic oil, a hydrocarbon oil, analcohol, or the like. For instance, the plasticizer may comprisedecaline, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol,oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane,n-dodecane, or mixtures thereof. In one embodiment, the plasticizer maycomprise a C5-C12 hydrocarbon, such as a C5-C12 saturated hydrocarbon.For example, the plasticizer may comprise heptane, hexane, or the like.

In one embodiment, the polyethylene used to produce the particles canhave a relatively high molecular weight. In fact, one of the advantagesof the present disclosure is the ability to rapidly blend relativelyhigh molecular weight polyethylene particles with a plasticizer withoutforming gel specks. In one embodiment, the use of higher molecularweight polyethylene particles may be beneficial, especially inapplications where greater strength properties are needed or desired.For example, the polyethylene used to produce the particles can have amolecular weight of greater than about 500,000 g/mol, such as greaterthan about 1,000,000 g/mol, such as greater than about 1,500,000 g/mol,such as greater than about 2,000,000 g/mol, such as greater than about2,500,000 g/mol, such as greater than about 3,000,000 g/mol, such asgreater than about 3,500,000 g/mol, such as even greater than about4,000,000 g/mol. In one embodiment, the polyethylene used to produce theparticles comprises a Ziegler-Natta catalyzed ultrahigh molecular weightpolyethylene.

The present disclosure is also directed to polymer articles formed fromthe above polymer composition. The polymer articles can be producedthrough a gel extrusion or gel-spinning process. Polymer articles madein accordance with the present disclosure include fibers, films,membranes, or the like.

The present disclosure is also directed to a process for producingpolymer articles. The process includes the steps of forming a gel-likecomposition from the polymer composition described above. The gel-likecomposition is then extruded through a die to form a polymer article.The polymer article, for instance, may comprise fibers, a film, or amembrane. During formation of the polymer article, at least part of theplasticizer is separated and removed from the polyethylene particle. Forinstance, in one embodiment, greater than 80%, such as greater than 90%,such as greater than 95%, such as greater than 98% of the plasticizer isremoved during formation of the polymer article.

Other features and aspects of the present disclosure are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present disclosure is set forthmore particularly in the remainder of the specification, includingreference to the accompanying figures, in which:

FIG. 1 is a graphical representation of some of the results obtained inExample No. 1 below.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentdisclosure.

In general, the present disclosure is directed to a polymer compositionwell suited for producing gel extruded articles, such as fibers, filmsand membranes. The polymer composition contains a polyethylene resin,such as high density polyethylene particles, combined with aplasticizer. In accordance with the present disclosure, the polyethyleneparticles are particularly constructed so that the particles rapidlyform a homogeneous gel-like material when combined with the plasticizerand heated.

In the past, various problems have been experienced in gel processingpolyethylene polymers. For instance, various polyethylene polymers canrequire significant amounts of time in order to dissolve into aprocessing solvent during extrusion. If the residence time in theextruder is not long enough to allow complete dissolution of the polymerresin, the formation of defects can result in the polymer article beingformed. The defects, which can occur due to the presence of gel specks,can lead to difficulties in production of the articles or reducedproduct quality. Ultimately, longer dissolution times or incompletedissolution can limit product capability. In addition, these problemscan result in lower capacity of a gel-processing production line.

The present disclosure, however, is directed to selecting a particularpolyethylene resin that is better suited for quickly blending with aplasticizer to form a homogeneous gel-like material. In particular, itis believed that the polyethylene particles of the present disclosureprovide higher interaction area between the plasticizer and the polymer,lowering solubility times thereby eliminating gel specks and preventingdefects from occurring in the extruded polymer articles. In oneembodiment, the process of the present disclosure allows for the use ofrelatively high molecular weight polymers that can result in polymerarticles with improved physical properties, such as strengthcharacteristics.

In accordance with the present disclosure, the polyethylene resinselected for combination with the plasticizer can have at least one oftwo physical characteristics. In one embodiment, for instance, the resinis made from a polyethylene polymer having a relatively low bulkdensity. The lower bulk density has been found to unexpectedly anddramatically shorten the time needed for the polymer to dissolve in theplasticizer or otherwise form a homogeneous gel-like material. In analternative embodiment or in addition to having a relatively low bulkdensity, the polyethylene resin particles can have a unique particlesize distribution that has also been found to dramatically improvedissolution times.

According to the present disclosure, the polymer composition contains apolyethylene polymer. As used herein, a polyethylene polymer refers to apolymer made from over 90% ethylene derived units, such as greater than95% ethylene derived units, or 100% ethylene derived units. Thepolyethylene can be a homopolymer or a copolymer, including aterpolymer, having other monomeric units. In one embodiment, thepolyethylene particles are made from a high density polyethylene. A highdensity polyethylene has a density of about 0.93 g/cm³ or greater. Thepolyethylene used to produce the particles can comprise a high molecularweight polyethylene, a very high molecular weight polyethylene, and/oran ultrahigh molecular weight polyethylene. “High molecular weightpolyethylene” refers to polyethylene compositions with weight-averagemolecular weight of at least about 3×10⁵ g/mol and, as used herein, isintended to include very-high molecular weight polyethylene andultra-high molecular weight polyethylene. For purposes of the presentspecification, the molecular weights referenced herein are determined inaccordance with the Margolies equation (“Margolies molecular weight”).

“Very-high molecular weight polyethylene” refers to polyethylenecompositions with a weight average molecular weight of less than about3×10⁶ g/mol and more than about 1×10⁶ g/mol. In some embodiments, themolecular weight of the very-high molecular weight polyethylenecomposition is between about 2×10⁶ g/mol and less than about 3×10⁶g/mol.

“Ultra-high molecular weight polyethylene” refers to polyethylenecompositions with weight-average molecular weight of at least about3×10⁶ g/mol. In some embodiments, the molecular weight of the ultra-highmolecular weight polyethylene composition is between about 3×10⁶ g/moland about 30×10⁶ g/mol, or between about 3×10⁶ g/mol and about 20×10⁶g/mol, or between about 3×10⁶ g/mol and about 10×10⁶ g/mol, or betweenabout 3×10⁶ g/mol and about 6×10⁶ g/mol.

As described above, in one embodiment, the polyethylene is a homopolymerof ethylene. In another embodiment, the polyethylene may be a copolymer.For instance, the polyethylene may be a copolymer of ethylene andanother olefin containing from 3 to 16 carbon atoms, such as from 3 to10 carbon atoms, such as from 3 to 8 carbon atoms. These other olefinsinclude, but are not limited to, propylene, 1-butene, 1-pentene,1-hexene, 1-heptene, 1-octene, 4-methylpent-1-ene, 1-decene, 1-dodecene,1-hexadecene and the like. Also utilizable herein are polyene comonomerssuch as 1,3-hexadiene, 1,4-hexadiene, cyclopentadiene,dicyclopentadiene, 4-vinylcyclohex-1-ene, 1,5-cyclooctadiene,5-vinylidene-2-norbomene and 5-vinyl-2-norbomene. However, when present,the amount of the non-ethylene monomer(s) in the copolymer may be lessthan about 10 mol. %, such as less than about 5 mol. %, such as lessthan about 2.5 mol. %, such as less than about 1 mol. %, wherein themol. % is based on the total moles of monomer in the polymer.

In one embodiment, the polyethylene may have a monomodal molecularweight distribution. Alternatively, the polyethylene may exhibit abimodal molecular weight distribution. For instance, a bimodaldistribution generally refers to a polymer having a distinct highermolecular weight and a distinct lower molecular weight (e.g. twodistinct peaks) on a size exclusion chromatography or gel permeationchromatography curve. In another embodiment, the polyethylene mayexhibit more than two molecular weight distribution peaks such that thepolyethylene exhibits a multimodal (e.g., trimodal, tetramodal, etc.)distribution. Alternatively, the polyethylene may exhibit a broadmolecular weight distribution wherein the polyethylene is comprised of ablend of higher and lower molecular weight components such that the sizeexclusion chromatography or gel permeation chromatography curve does notexhibit at least two distinct peaks but instead exhibits one distinctpeak broader than the individual component peaks.

In one embodiment, the composition may be comprised of more than onepolyethylene, each having a different molecular weight and/or molecularweight distribution. For instance, the molecular weight distribution maybe within the average molecular weight specifications provided above.

In addition, the composition may be comprised of a blend of one or morepolyethylene polymers or copolymers and another thermoplastic polymersuch as a polypropylene, a polybutylene, a polymethylpentene, a linearlow density polyethylene, or mixtures thereof. However, the amount ofnon-polyethylene polymer(s) in the composition may be less than about 10wt. %, such as less than about 5 wt. %, such as less than about 2.5 wt.%, such as less than about 1 wt. %, wherein the wt % is based on thetotal weight of the composition.

Any method known in the art can be utilized to synthesize thepolyethylene. The polyethylene powder is typically produced by thecatalytic polymerization of ethylene monomer or optionally with one ormore other 1-olefin co-monomers, the 1-olefin content in the finalpolymer being less or equal to 10% of the ethylene content, with aheterogeneous catalyst and an organo aluminum or magnesium compound ascocatalyst. The ethylene is usually polymerized in gaseous phase orslurry phase at relatively low temperatures and pressures. Thepolymerization reaction may be carried out at a temperature of between50° C. and 100° C. and pressures in the range of 0.02 and 2 MPa.

The molecular weight of the polyethylene can be adjusted by addinghydrogen. Altering the temperature and/or the type and concentration ofthe co-catalyst may also be used to fine tune the molecular weight.Additionally, the reaction may occur in the presence of antistaticagents to avoid fouling and product contamination.

Suitable catalyst systems include but are not limited to Ziegler-Nattatype catalysts. Typically Ziegler-Natta type catalysts are derived by acombination of transition metal compounds of Groups 4 to 8 of thePeriodic Table and alkyl or hydride derivatives of metals from Groups 1to 3 of the Periodic Table. Transition metal derivatives used usuallycomprise the metal halides or esters or combinations thereof. ExemplaryZiegler-Natta catalysts include those based on the reaction products oforgano aluminum or magnesium compounds, such as for example but notlimited to aluminum or magnesium alkyls and titanium, vanadium orchromium halides or esters. The heterogeneous catalyst might be eitherunsupported or supported on porous fine grained materials, such assilica or magnesium chloride. Such support can be added during synthesisof the catalyst or may be obtained as a chemical reaction product of thecatalyst synthesis itself.

As will be explained in greater detail below, of particular advantage,the polyethylene particles made in accordance with the presentdisclosure can quickly blend or dissolve with the plasticizer even whenthe polyethylene polymer is produced using a Ziegler-Natta catalyst. Inthe past, for instance, problems have been experienced in rapidlyforming homogeneous gel-like materials for gel processing usingZiegler-Natta catalyzed polyethylene polymers, especially high molecularweight polymers.

In one embodiment, a suitable catalyst system could be obtained by thereaction of a titanium(IV) compound with a trialkyl aluminum compound inan inert organic solvent at temperatures in the range of −40° C., to100° C., preferably −20° C. to 50° C. The concentrations of the startingmaterials are in the range of 0.1 to 9 mol/L, preferably 0.2 to 5 mol/L,for the titanium(IV) compound and in the range of 0.01 to 1 mol/L,preferably 0.02 to 0.2 mol/L for the trialkyl aluminum compound. Thetitanium component is added to the aluminum component over a period of0.1 min to 60 min, preferably 1 min to 30 min, the molar ratio oftitanium and aluminum in the final mixture being in the range of 1:0.01to 1:4.

In another embodiment, a suitable catalyst system is obtained by a oneor two-step reaction of a titanium(IV) compound with a trialkyl aluminumcompound in an inert organic solvent at temperatures in the range of−40° C. to 200° C., preferably −20° C. to 150° C. In the first step thetitanium(IV) compound is reacted with the trialkyl aluminum compound attemperatures in the range of −40° C. to 100° C., preferably −20° C. to50° C. using a molar ratio of titanium to aluminum in the range of 1:0.1to 1:0.8. The concentrations of the starting materials are in the rangeof 0.1 to 9.1 mol/L, preferably 5 to 9.1 mol/L, for the titanium(IV)compound and in the range of 0.05 and 1 mol/L, preferably 0.1 to 0.9mol/L for the trialkyl aluminum compound. The titanium component isadded to the aluminum compound over a period of 0.1 min to 800 min,preferably 30 min to 600 min. In a second step, if applied, the reactionproduct obtained in the first step is treated with a trialkyl aluminumcompound at temperatures in the range of −10° C. to 150° C., preferably10° C. to 130° C. using a molar ratio of titanium to aluminum in therange of 1:0.01 to 1:5.

In yet another embodiment, a suitable catalyst system is obtained by aprocedure wherein, in a first reaction stage, a magnesium alcoholate isreacted with a titanium chloride in an inert hydrocarbon at atemperature of 50° to 100° C. In a second reaction stage the reactionmixture formed is subjected to heat treatment for a period of about 10to 100 hours at a temperature of 110° to 200° C. accompanied byevolution of alkyl chloride until no further alkyl chloride is evolved,and the solid is then freed from soluble reaction products by washingseveral times with a hydrocarbon.

In a further embodiment, catalysts supported on silica, such as forexample the commercially available catalyst system Sylopol 5917 can alsobe used.

Using such catalyst systems, the polymerization is normally carried outin suspension at low pressure and temperature in one or multiple steps,continuous or batch. The polymerization temperature is typically in therange of 30° C. to 130° C., preferably is the range of 50° C. and 90° C.and the ethylene partial pressure is typically less than 10 MPa,preferably 0.05 and 5 MPa. Trialkyl aluminums, like for example but notlimited to isoprenyl aluminum and triisobutyl aluminum, are used asco-catalyst such that the ratio of Al:Ti (co-catalyst versus catalyst)is in the range of 0.01 to 100:1, more preferably is the range of 0.03to 50:1. The solvent is an inert organic solvent as typically used forZiegler type polymerizations. Examples are butane, pentane, hexane,cyclohexene, octane, nonane, decane, their isomers and mixtures thereof.The polymer molecular mass is controlled through feeding hydrogen. Theratio of hydrogen partial pressure to ethylene partial pressure is inthe range of 0 to 50, preferably the range of 0 to 10. The polymer isisolated and dried in a fluidized bed drier under nitrogen. The solventmay be removed through steam distillation in case of using high boilingsolvents. Salts of long chain fatty acids may be added as a stabilizer.Typical examples are calcium, magnesium and zinc stearate.

Optionally, other catalysts such as Phillips catalysts, metallocenes andpost metallocenes may be employed. Generally a cocatalyst such asalumoxane or alkyl aluminum or alkyl magnesium compound is alsoemployed. Other suitable catalyst systems include Group 4 metalcomplexes of phenolate ether ligands.

In accordance with the present disclosure, the polyethylene polymer isformed into particles and combined with a plasticizer. In order todramatically and unexpectedly increase dissolution rates into theplasticizer and/or to rapidly form a homogeneous gel-like material, thepolyethylene is particularly selected so as to have at least one of twophysical characteristics. In one embodiment, for instance, thepolyethylene particles are made from a polyethylene polymer having arelatively low bulk density as measured according to DIN53466. Forinstance, in one embodiment, the bulk density is generally less thanabout 0.4 g/cm³, such as less than about 0.35 g/cm³, such as less thanabout 0.33 g/cm³, such as less than about 0.3 g/cm³, such as less thanabout 0.28 g/cm³, such as less than about 0.26 g/cm³. The bulk densityis generally greater than about 0.1 g/cm³, such as greater than about0.15 g/cm³. In one embodiment, the polymer has a bulk density of fromabout 0.2 g/cm³ to about 0.27 g/cm³.

Alternatively or in addition to having a relatively low bulk density,the polyethylene particles can have a controlled particle sizedistribution that has also been found to dramatically improvedissolution times into the plasticizer when heated. In one embodiment,for instance, the polyethylene particles can be a free-flowing powder.In accordance with the present disclosure, the particles can have amedian particle size (d50) of less than 125 microns. For example, themedian particle size (d50) of the polyethylene particles can be lessthan about 110 microns, such as less than about 105 microns, such asless than about 100 microns, such as less than about 95 microns. Themedian particle size (d50) is generally greater than about 60 microns.For instance, the median particle size (d50) can be from about 60microns to less than 125 microns, such as from about 70 microns to about110 microns. The powder particle size can be measured utilizing a laserdiffraction method according to ISO 13320.

In addition to having a median particle size within the above ranges,the particle size distribution of the polyethylene polymer particles canalso be controlled so as to contain relatively little to no largerparticles. For instance, in one embodiment, 90% of the polyethyleneparticles can have a particle size of less than about 180 microns. Inother embodiments, 90% of the polyethylene particles can have a particlesize of less than about 170 microns, such as less than about 165microns, such as less than about 160 microns, such as less than about155 microns, such as less than about 150 microns, such as less thanabout 145 microns, such as less than about 140 microns, such as lessthan about 135 microns, such as less than about 130 microns, such asless than about 125 microns, such as less than about 120 microns.

The above physical characteristics have been found to dramaticallyimprove the ability of the polymer particles to blend with theplasticizer and form a homogeneous solution during gel processing. Inone embodiment, the polyethylene polymer selected for use in the polymercomposition can have a relatively high molecular weight. For instance,the molecular weight can be relatively high in relation to the bulkdensity of the polymer. Of particular advantage, it was discovered thateven relatively high molecular weight polymers can rapidly blend withthe plasticizer and have extremely short dissolution times. In someapplications, for instance, the use of relatively high molecular weightpolymers may be preferred. The use of high molecular weight polymers,for instance, may improve various physical properties of the resultingproduct, such as the strength characteristics of the resulting product.

The polyethylene polymer, for instance, may have an average molecularweight, as determined according to the Margolies equation. The molecularweight can be determined by first measuring the viscosity numberaccording to DIN EN ISO Test 1628. Dry powder flow is measured using a25 mm nozzle. The molecular weight is then calculated using theMargolies equation from the viscosity numbers, of at least or greaterthan about 500,000 g/mol, such as greater than about 1,000,000 g/mol,such as greater than about 1,500,000 g/mol, such as greater than about2,000,000 g/mol, such as greater than about 2,500,000 g/mol, such asgreater than about 3,000,000 g/mol, such as greater than about 3,500,000g/mol, such as greater than about 4,000,000 g/mol. The average molecularweight is generally less than about 12,000,000 g/mol, such as less thanabout 10,000,000.

The polyethylene may have a viscosity number of from at least 100 mL/g,such as at least 500 mL/g, such as at least 1,500 mL/g, such as at least2,000 mL/g, such as at least 4,000 mL/g to less than about 6,000 mL/g,such as less than about 5,000 mL/g, such as less than about 4000 mL/g,such as less than about 3,000 mL/g, such as less than about 1,000 mL/g,as determined according to ISO 1628 part 3 utilizing a concentration indecahydronaphthalene of 0.0002 g/mL.

The polyethylene may have a crystallinity of from at least about 40% to85%, such as from 45% to 80%.

In order to form polymer articles through a gel spinning or extrudingprocess, the polyethylene particles as described above are combined witha plasticizer to form a polymer composition. In general, thepolyethylene particles are present in the polymer composition in anamount up to about 50% by weight. For instance, the polyethyleneparticles can be present in the polymer composition in an amount lessthan about 45% by weight, such as in an amount less than about 40% byweight, such as in an amount less than about 35% by weight, such as inan amount less than about 30% by weight, such as in an amount less thanabout 25% by weight, such as in an amount less than about 20% by weight,such as in an amount less than about 15% by weight, such as in an amountless than about 10% by weight, such as in an amount less than about 5%by weight. The polyethylene particles can be present in the compositionin an amount greater than about 1% by weight, such as in an amountgreater than about 3% by weight, such as in an amount greater than about5% by weight, such as in an amount greater than about 10% by weight,such as in an amount greater than about 15% by weight, such as in anamount greater than about 20% by weight, such as in an amount greaterthan about 25% by weight. During gel processing, the plasticizer can besubstantially or completely removed in forming polymer articles. Forexample, in one embodiment, the resulting polymer article can containthe polyethylene polymer in an amount greater than about 70% by weight,such as in an amount greater than about 80% by weight, such as in anamount greater than about 85% by weight, such as in an amount greaterthan about 90% by weight, such as in an amount greater than about 95% byweight, such as in an amount greater than about 98% by weight, such asin an amount greater than about 99% by weight.

In general, any suitable plasticizer can be combined with thepolyethylene particles as long as the plasticizer is capable of forminga gel-like material suitable for gel spinning or extruding. Theplasticizer, for instance, may comprise a hydrocarbon oil, an alcohol,an ether, an ester such as a diester, or mixtures thereof. For instance,suitable plasticizers include mineral oil, a paraffinic oil, decaline,and the like. Other plasticizers include xylene, dioctyl phthalate,dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonylalcohol, diphenyl ether, n-decane, n-dodecane, octane, nonane, kerosene,toluene, naphthalene, tetraline, and the like. In one embodiment, theplasticizer may comprise a halogenated hydrocarbon, such asmonochlorobenzene. Cycloalkanes and cycloalkenes may also be used, suchas camphene, methane, dipentene, methylcyclopentadiene, tricyclodecane,1,2,4,5-tetramethyl-1,4-cyclohexadiene, and the like. The plasticizermay comprise mixtures and combinations of any of the above as well.

The plasticizer is generally present in the composition used to form thepolymer articles in an amount greater than about 50% by weight, such asin an amount greater than about 55% by weight, such as in an amountgreater than about 60% by weight, such as in an amount greater thanabout 65% by weight, such as in an amount greater than about 70% byweight, such as in an amount greater than about 75% by weight, such asin an amount greater than about 80% by weight, such as in an amountgreater than about 85% by weight, such as in an amount greater thanabout 90% by weight, such as in an amount greater than about 95% byweight, such as in an amount greater than about 98% by weight. In fact,the plasticizer can be present in an amount up to about 99.5% by weight.

When the polyethylene particles made in accordance with the presentdisclosure are combined with a plasticizer, the particles rapidly blendwith the plasticizer to form a homogeneous gel-like material. Forexample, polyethylene particles made according to the presentdisclosure, when tested according to the solubility test as described inthe examples below, can have a solubility of less than about 3 mins.,such as less than about 2.5 mins., such as less than about 2 mins., suchas even less than about 1.8 mins. The solubility is generally greaterthan about 0.1 mins.

In order to form polymer articles in accordance with the presentdisclosure, the polyethylene particles are combined with the plasticizerand extruded through a die of a desired shape. In one embodiment, thecomposition can be heated within the extruder. For example, theplasticizer can be combined with the polyethylene particles and fed intoan extruder. In accordance with the present disclosure, the plasticizerand polyethylene particles form a homogeneous gel-like material prior toleaving the extruder for forming polymer articles with little to noimpurities.

In one embodiment, elongated articles are formed during the gel spinningor extruding process. The polymer article, for instance, may be in theform of a fiber, a film, or a membrane.

During the process, at least a portion of the plasticizer is removedfrom the final product. The plasticizer removal process may occur due toevaporation when a relatively volatile plasticizer is used. Otherwise,an extraction liquid can be used to remove the plasticizer. In oneembodiment, evaporation and extraction are both used.

If desired, the resulting polymer article can be stretched at anelevated temperature below the melting point of the polyethylene polymerto increase strength and modulus. Suitable temperatures for stretchingare in the range of from about ambient temperature to about 155° C. Thedraw ratios can generally be greater than about 4, such as greater thanabout 6, such as greater than about 8, such as greater than about 10,such as greater than about 15, such as greater than about 20, such asgreater than about 25, such as greater than about 30. In certainembodiments, the draw ratio can be greater than about 50, such asgreater than about 100, such as greater than about 110, such as greaterthan about 120, such as greater than about 130, such as greater thanabout 140, such as greater than about 150. Draw ratios are generallyless than about 1,000, such as less than about 800, such as less thanabout 600, such as less than about 400. In one embodiment, lower drawratios are used such as from about 4 to about 10. The polymer articlecan be uniaxially stretched or biaxially stretched.

Polymer articles made in accordance with the present disclosure havenumerous uses and applications. For example, in one embodiment, theprocess is used to produce a membrane. The membrane can be used, forinstance, as a battery separator. Alternatively, the membrane can beused as a microfilter. When producing fibers, the fibers can be used toproduce nonwoven fabrics, ropes, nets, and the like. In one embodiment,the fibers can be used as a filler material in ballistic apparel.

The polymer composition and polymer articles made in accordance with thepresent disclosure may contain various other additives, such as heatstabilizers, light stabilizers, UV absorbers, flame retardants,lubricants, colorants, acid scavengers, and the like.

In one embodiment, a heat stabilizer may be present in the composition.The heat stabilizer may include, but is not limited to, phosphites,aminic antioxidants, phenolic antioxidants, or any combination thereof.

In one embodiment, an antioxidant may be present in the composition. Theantioxidant may include, but is not limited to, secondary aromaticamines, benzofuranones, sterically hindered phenols, or any combinationthereof.

In one embodiment, a light stabilizer may be present in the composition.The light stabilizer may include, but is not limited to,2-(2′-hydroxyphenyl)-benzotriazoles, 2-hydroxy-4-alkoxybenzophenones,nickel containing light stabilizers,3,5-di-tert-butyl-4-hydroxbenzoates, sterically hindered amines (HALS),or any combination thereof.

In one embodiment, a UV absorber may be present in the composition inlieu of or In addition to the light stabilizer. The UV absorber mayinclude, but is not limited to, a benzotriazole, a benzoate, or acombination thereof, or any combination thereof.

In one embodiment, a halogenated flame retardant may be present in thecomposition. The halogenated flame retardant may include, but is notlimited to, tetrabromobisphenol A (TBBA), tetrabromophthalic acidanhydride, dedecachloropentacyclooctadecadiene (dechlorane),hexabromocyclodedecane, chlorinated paraffins, or any combinationthereof.

In one embodiment, a non-halogenated flame retardant may be present inthe composition. The non-halogenated flame retardant may include, but isnot limited to, resorcinol diphosphoric acid tetraphenyl ester (RDP),ammonium polyphosphate (APP), phosphine acid derivatives, triarylphosphates, trichloropropylphosphate (TCPP), magnesium hydroxide,aluminum trihydroxide, antimony trioxide.

In one embodiment, a lubricant may be present in the composition. Thelubricant may include, but is not limited to, silicone oil, waxes,molybdenum disulfide, or any combination thereof.

In one embodiment, a colorant may be present in the composition. Thecolorant may include, but is not limited to, inorganic and organic basedcolor pigments.

In one embodiment, an acid scavenger may be present in the composition.One example of an acid scavenger, for instance, is calcium stearate.

These additives may be used singly or in any combination thereof. Ingeneral, unless stated otherwise, if the additives are utilized, theymay be present in an amount of at least about 0.05 wt. %, such as atlast about 0.1 wt. %, such as at least about 0.25 wt. %, such as atleast about 0.5 wt. %, such as at least about 1 wt. % and generally lessthan about 20 wt. %, such as less than about 10 wt. %, such as less thanabout 5 wt. %, such as less than about 4 wt. %, such as less than about2 wt. %. The sum of the wt. % of all of the components, including anyadditives if present, utilized in the polymer composition will be 100wt. %.

The present disclosure may be better understood with reference to thefollowing examples. The following examples are given below by way ofillustration and not by way of limitation. The following experimentswere conducted in order to show some of the benefits and advantages ofthe present invention.

Example 1

Three grades of high density polyethylene were selected with similarmolecular weights (4-4.5 million g/mol) and different bulk densities.The samples were sieved such that a middle fraction was received of100-125 micrometers grain size. Accordingly, comparative values ofmolecular weight and grain size were obtained with differing values ofbulk density. Bulk densities were determined by International StandardISO 60.

Solubility testing was performed to determine dissolving time in mineraloil.

Solubility Test

The following solubility test was used to determine dissolving time forthe different samples. The polymer particles were combined with the oilunder defined conditions and while monitoring the torque put on thescrew.

When the resin dissolves in the oil, the polymer chains increase theviscosity of the fluid (same principle as VN-testing) leading to anincrease in torque. When all resin is dissolved in the oil, the torquereaches an equilibrium level. The time that is needed to reachequilibrium torque is a measure for the dissolving time of the resin.

Equipment Used: Haake RheoStress 600 equipped with screw-like stirrer.

Sample Preparation:

1. Weight 80 g of process oil, 450 mg of Irganox B215 and 225 mg ofresin powder into measurement bowl.2. Install bowl to Rheostress and start measurement program.

Test Program:

1. Heat bowl to 180° C.2. Once temperature is reached, set screw rpm to 130 rpm.3. Keep stirring for 150 min while recording torque reading.

Data Evaluation:

1. Identify region where torque (viscosity) is constant.2. Generate linear fit (horizontal line) in this region (typically:140-160 minutes).3. Calculate confidence interval for the fit line.4. Identify first data point within the confidence interval to determinedissolving time.5. For end viscosity, determine average viscosity in linear fit region.

Viscosity testing of the four samples are illustrated in FIG. 1.Dissolution times are shown in the following table.

TABLE 1 Property Sample No. 1 Sample No. 2 Sample No. 3 Average 4.7 4.04.6 molecular-weight (million g/mol) Grain size (μm) 100-125 100-125100-125 Bulk density 0.45 0.25 0.25 (g/cm³) Dissolving time 11.6 4.0 3.2(min)

As shown above, the lower bulk density samples unexpectedly anddramatically reduced the dissolving time.

Example 2

The following example was performed in order to demonstrate thatparticle size distribution can have a dramatic effect on dissolvingtime.

Two different samples of high density polyethylene powders wereselected. Sample No. 1 had an average particle size (d50) of 98 microns,while Sample No. 2 had an average particle size (d50) of 135 microns.Except for the particle size distribution, the polyethylene polymersused in the samples were relatively the same.

The different samples were subjected to the solubility test as describedabove. In addition, the polyethylene particles were gel extruded and theresulting polymer articles were tested for various properties. Thefollowing results were obtained:

TABLE 2 Sample Sample Property Unit Test Method No. 1 No. 2 Averageg/mol Calculated from 1.6 * 10⁶ 1.7 * 10⁶ molecular weight VN usingMargolies' equation Viscosity number ml/g ISO 1628, part 3 1040 1100Intrinsic viscosity ml/g ISO 1628, part 3 977 1000 Bulk density g/cm³ISO 60 0.46 0.45 Average particle μm Laser scattering 98 135.0 size(d50) Average particle μm Laser scattering 158 200 size (d90) DissolvingTime mins. <2 mins. 4 mins.

As shown above, the different particle size distribution of Sample No. 1reduced the dissolving time by more than 50% in relation to Sample No.2.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

What is claimed:
 1. A polymer composition for producing gel extrudedarticles comprising: a plasticizer; and high density polyethyleneparticles combined with the plasticizer, the high density polyethyleneparticles having at least one of the following characteristics: (a) abulk density of less than about 0.35 g/cm³; and/or (b) a median particlesize (d50) of less than 125 microns and wherein 90% of the particleshave a particle size of less than about 180 microns, and wherein thehigh density polyethylene has an average molecular weight of greaterthan about 1,500,000 g/mol.
 2. A polymer composition as defined in claim1, wherein the high density polyethylene particles have a bulk densityof less than about 0.33 g/cm³.
 3. A polymer composition as defined inclaim 1, wherein the high density polyethylene particles have a bulkdensity of less than about 0.3 g/cm³ and greater than about 0.15 g/cm³.4. A polymer composition as defined in claim 1, wherein the high densitypolyethylene particles have a median particle size (d50) of less thanabout 100 microns.
 5. A polymer composition as defined in claim 1,wherein the high density polyethylene particles have a median particlesize (d50) of from about 60 microns to less than 110 microns.
 6. Apolymer composition as defined in claim 1, wherein 90% of the highdensity polyethylene particles have a particle size of less than about170 microns.
 7. A polymer composition as defined in claim 1, wherein,when tested according to a solubility test, the high densitypolyethylene particles have a solubility of less than about 3 mins.
 8. Apolymer composition as defined in claim 1, wherein the high densitypolyethylene particles possess both of the characteristics (a) and (b).9. A polymer composition as defined in claim 1, wherein the high densitypolyethylene particles are present in the composition in an amount up toabout 50% by weight.
 10. A polymer composition as defined in claim 1,wherein the plasticizer comprises mineral oil, a paraffinic oil, ahydrocarbon, an alcohol, an ether, an ester, or mixtures thereof.
 11. Apolymer composition as defined in claim 1, wherein the high densitypolyethylene has a molecular weight of greater than about 2,000,000g/mol.
 12. A polymer composition as defined in claim 1, wherein the highdensity polyethylene is a Ziegler-Natta catalyzed ultrahigh molecularweight polyethylene.
 13. A polymer composition as defined in claim 1,wherein the plasticizer comprises decaline, xylene, dioctyl phthalate,dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonylalcohol, diphenyl ether, n-decane, n-dodecane, octane, nonane, kerosene,toluene, naphthalene, tetraline, monochlorobenzene, camphene, methane,dipentene, methylcyclopentadiene, tricyclodecane,1,2,4,5-tetramethyl-1,4-cyclohexadiene, or mixtures thereof.
 14. Aprocess for producing polymer articles comprising: forming the polymercomposition as defined in claim 1 into a gel-like composition; extrudingthe gel-like composition through a die to form a polymer article, thepolymer article comprising fibers, a film, or a membrane.
 15. A processas defined in claim 14, further comprising the step of removing at leastpart of the plasticizer from the polymer article.
 16. A polymer articlecomprising a fiber, a film, or a membrane, the polymer article beingproduced by: combining a plasticizer with high density polyethyleneparticles to form a gel-like composition, the high density polyethyleneparticles having at least one of the following characteristics: (a) bulkdensity of less than about 0.35 g/cm³; and/or (b) a median particle size(d50) of less than 125 microns and wherein 90% of the particles have aparticle size of less than about 180 microns, and wherein the highdensity polyethylene has an average molecular weight of greater thanabout 1,500,000 g/mol; and extruding the gel-like composition through adie to form the polymer article.
 17. A polymer article as defined inclaim 16, wherein the high density polyethylene particles have a bulkdensity of less than about 0.3 g/cm³ and greater than about 0.15 g/cm³.18. A polymer article as defined in claim 16, wherein the high densitypolyethylene particles have a median particle size (d50) of from about60 microns to less than 110 microns.
 19. A polymer article as defined inclaim 16, wherein the high density polyethylene particles possess bothof the characteristics (a) and (b).
 20. A polymer article as defined inclaim 16, wherein the polymer article comprises a membrane.
 21. Apolymer article as defined in claim 16, wherein the polymer articlecomprises fibers.